<p>The mechanical properties of reinforced nanocomposites depend on several factors, including the shape of the nanoparticles, their interface area with the polymer matrix, and how they are distributed. This study focuses on examining how the geometric shape of nanoparticles—specifically graphene nanoplatelets (GNPs), carbon nanotubes (CNTs), and icosahedral particles—affects the amount of debonded surfaces and the plasticity behavior of nanocomposites caused by debonding. To achieve this, the concept of an RVE with a random and isotropic distribution of elastic nanoparticles was used. Additionally, to improve accuracy, the effect of RVE size on the amount of debonded surfaces and the stress–strain curve was investigated simultaneously to determine the appropriate RVE size, taking into account computational costs. A constitutive model for the polymer matrix was employed, incorporating hyperelastic behavior with softening effects. The interface area was modeled using the cohesive zone model, with parameters derived from molecular dynamics results for carbon-based nanoparticles, as reported in the literature. When comparing planar shapes (GNP) to cylindrical (CNT) with the same aspect ratio, GNPs showed less debonding due to their larger planar surface area. While assuming perfect bonding, CNT has a greater effect on improving mechanical properties. Icosahedral nanoparticles, having the lowest surface-to-volume ratio (~ 1), exhibited the highest debonding rate at lower strains, while CNTs (~ 28) and GNPs (~ 35) delayed debonding and improved mechanical performance. In nanocomposites based on CNTs and GNPs, functionalization significantly increases cohesive strength but has minimal impact on icosahedral nanoparticles due to their limited surface contact.</p>